In these leisurely essays, Wilkinson ( Big Sugar ) writes about persons who earn their living on the water. The ``riverkeeper'' of the title is John Cronin, who patrols the Hudson River, tracking down polluters (his most notorious catch was the Exxon Corporation). Another piece examines the customs of Portuguese fishermen in Provincetown, Mass. The longest essay is devoted to the Tlingit Indians of Angoon, on Alaska's Admiralty Island: under U.S. government rule since 1882, when the American navy bombarded their tiny village, they have nonetheless determined to preserve their traditional way of life. Wilkinson's choppy prose style and his tendency to linger on insignificant details can be annoying, yet those curious about little-known aspects of American culture will find much of interest here. (Aug.)